1. Field of the Invention
This invention relates to an improvement in a process for anodizing an Si-based aluminum alloy to form an anodized film on a surface of the alloy.
2. Description of the Prior Art
In recent years, pistons of Si- or silicon-based aluminum alloy have been widely used in internal combustion engines because they are light in weight and hence can make reciprocal movements at a high speed.
A typical example of such pistons is schematically shown in FIG. 5 hereof. As illustrated therein, a piston 100 is disposed within a cylinder 101 such that it can make reciprocal movements at a high speed between top and bottom dead centers 102, 103 along an inner peripheral surface 104 of the cylinder 101. The piston 100 includes piston rings 100b which act to normally keep the piston 100 out of contact with the cylinder 101. However, the piston 100 occasionally inclines for certain reasons. When the piston 100 inclines more than a given amount, a skirt 100a of the piston 100 is brought into contact with the cylinder 101 to thereby prevent further inclination of the piston 100. Consequently, it becomes necessary for the piston 100, particularly the skirt 100a, to have excellent wear resistance.
An example of pistons which are light in weight and have excellent wear resistance may be one cast from an Si-based aluminum alloy with an anodized film formed on a surface thereof. Discussion will be made next as to a process for anodizing a surface of such a piston of Si-based aluminum alloy.
The anodization process comprises the steps of immersing the Si-based aluminum alloy piston into an electrolyte to make the piston act as an anode, charging the electrolyte with a direct current to electrolyze water therein to thereby generate oxygen, and causing the generated oxygen to react with aluminum to thereby form a film of Al2O3 on a surface of the Si-based aluminum alloy piston. The film of Al2O3 is passive and generally called an anodized film having good corrosion and wear resistance.
When a surface of the skirt that may be held in contact with the cylinder is rough, there is a fear that undesired scoring and burning will occur during movement of the piston. To prevent this burning, a certain proposed piston has an anodized film with a resin infiltrated into the film to reduce its friction resistance.
Discussion will be made next as to an anodized film with a resin infiltrated thereinto, with reference to FIGS. 6A to 6C showing a conventional piston of Si-based aluminum alloy.
An anodized film shown as an example in FIG. 6A is formed by using a sulfuric acid electrolyte. An aluminum alloy piston 100 as a base material includes Si particles 111 distributed therein. Si particles 112 located closely to a surface of the piston 100 adversely affect an anodized film 113, thereby making the anodized film uneven or rough.
In that part of the piston where an Si particle 115 is partially exposed to outside accidentally as shown in FIG. 6B, there is formed no anodized film but a large hollow D1. In that part of the piston where an Si particle 116 is positioned closely to a piston surface, there is formed an anodized film 117 but the film thickness is small compared to the surrounding film 113. In addition, a hollow D2 is formed above the film 117.
Consequently, it has been found that an even or flat anodized film can not be obtained by anodizing the aluminum alloy piston in a sulfuric acid electrolyte. It has also been found that microholes 118 resulted from a sulfuric acid electrolyte generally have a small hole diameter d1 of the order of 15 nm. In FIG. 6C, the microholes 118 are shown with a photosetting resin 119 in the form of a liquid, infiltrated thereinto and then photoset by applying a light thereto. Inherently, resins have small friction resistance. Thus, with the photoset resin 119 infiltrated into the anodized film portions 113, 117, it becomes possible to reduce friction resistance arising upon high-speed reciprocal movement of the aluminum alloy piston within the cylinder.
However, difficulty is experienced in making the anodized film 113 flat due to the hollows D1, D2 formed in the film 113 as shown in FIG. 6B. In addition, since the microholes 118 produced in the anodized film 113 have small hole diameters d1, it is difficult to make the film 113 contain the resin sufficiently. This leads to the fear that notwithstanding the resin 119 infiltrated into the anodized film 113, the friction resistance of the film will not be made as small as desired.
It is therefore an object of the present invention to provide an improved process for anodizing an Si-based aluminum alloy, which enables sufficient reduction of the friction resistance of an anodized film to be formed on the alloy.
According to an aspect of the present invention, there is provided a process for anodizing an Si-based aluminum alloy, which process comprises the steps of; subjecting the Si-based aluminum alloy to electrolysis in an electrolyte containing phosphate and fluoride to form an anodized film on the alloy; infiltrating a photosetting or thermosetting resin in liquid form into microholes in the anodized film; and radiating light or heat at the infiltrated resin to make the resin become hardened.
Phosphate causes the microholes to have large hole diameters while fluoride dissolves Si moderately and facilitates growth of the film. As a result, a large amount of the photosetting or thermosetting resin can be infiltrated into the microholes of the film, thereby making a surface of the film flat and thus reducing friction resistance of the film.
In a preferred form, the resin contains fluoride. Since fluoride has good wear- and heat-resisting properties, inclusion of fluoride makes the alloy best suited for application to pistons, which are exposed to a high temperature.